Abstract

The oxidation behavior of synthetic α-NiS in air has been investigated over the temperature range 670–700 °C. The α-NiS was ground and sieved to give a particle size ranging from 53 to 90 μm. Three oxidation paths were observed:

 
\[(i)\ {\alpha}-NiS\ +\ 3/2\ O_{2}\ {\rightarrow}\ NiO\ +\ SO_{2}\]
 
\[(ii)\ 3{\alpha}-NiS\ +O_{2}\ {\rightarrow}\ Ni_{3}S_{2}\ +\ SO_{2}\]
 
\[(iii)\ Ni_{3}S_{2}\ +\ 7/2\ O_{2}\ {\rightarrow}\ 3NiO\ +\ 2SO_{2}\]

No Ni3S2 (heazlewoodite) was observed over the course of α-NiS oxidation at 670 and 680 °C. The dominant oxidation path at this temperature is path i. At 700 °C, however, all three oxidation paths were observed. As an intermediate oxidation product, Ni3S2 steadily exsolved from α-NiS, reaching a maximum quantity after about 80 min of oxidation, declining afterward, and approaching annihilation at 160 min of oxidation. Experimental results show that the exsolution of Ni3S2 is likely triggered by the loss of one third of S in the α-NiS structure with the release of SO2 rather than by an intrinsic thermal decomposition of α-NiS to α-Ni1−xS + Ni3S2. The eventual annihilation of Ni3S2 was caused by a further oxidation of Ni3S2 to NiO. Oxidation paths 2 and 3 form a typical single chain reaction:

 
\[{\alpha}-NiS{{\rightarrow}_{}^{\mathit{k}_{1}}}\ Ni_{3}S_{2}\ {{\rightarrow}_{}^{\mathit{k}_{2}}}\ NiO\]

The approximate values of k1 are k2 are 3 × 10−4s−1 and 5 × 10−4s−1 respectively.

Oxidation temperature was found to play important roles both in the oxidation kinetics and the oxidation mechanism. By decreasing 10 °C from 680 to 670 °C, the average reaction rate (dy/dt, where y is the reaction extent) over the experiment time scale almost decreased to one third of its original rate (from 3.3 × 10−5s−1 to 1.2 × 10−5s−1). The reaction mechanism in the temperature range 670 to 680 °C is constant with Ea = 868.2 kJ/mol.

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